In recent years, technologies of optical fibers have achieved remarkable developments and have found a wide range of practical applications in fields such as communications, control systems, various optically measuring techniques and the like. Included among such technologies is, for example, a system in which signals are converted into optical signals and the optical signals are transmitted through optical fibers and received as changed to the contemplated type of signals.
An optical fiber is comprised of a core and a cladding surrounding the core. The core for an optical fiber usually has a refractive index of about 1.43 to about 1.60 and is produced by drawing quartz glass or like glass into threads or made of fibers of polymethyl methacrylate or like plastics. Generally the core has a diameter of about 5 to about 1000 .mu.m. The cladding is a transparent coating layer of about 3 to about 100 .mu.m thickness formed around the core and having a lower refractive index than the core.
Optical signals are transmitted through the core of an optical fiber, repeating total reflection at the interface between the core and the cladding during the transmission. For efficient transmission of optical signals through the optical fiber over a long distance with substantially no loss, cladding materials for optical fibers are required to have properties of being:
(1) lower in refractive index than the core, PA1 (2) excellent in adhesion to the core, PA1 (3) outstanding in flexibility, PA1 (4) not crystalline, PA1 (5) least light-absorbable, PA1 (6) not prone to thermal decomposition and thermal contraction, PA1 (7) scarcely variable in properties depending on temperatures, PA1 (8) high in water resistance and oil resistance, and PA1 (9) outstanding in curability and strength.
Conventional cladding materials include, for example, silicone resins, fluorine-containing resins, boron- or fluorine-containing quartz glass and the like. However, these materials are not satisfactory in properties. More specifically, silicone resins have the drawbacks of having a high refractive index and being prone to thermal decomposition and thermal contraction, widely variable in properties with temperatures and poor in curability and strength. Generally it is known to use as fluorine-containing resins non-crosslinkable thermoplastic resins such as a copolymer of vinylidene fluoride and tetrafluoroethylene (U.S. Pat. No. 3,930,103) and a fluoroalkyl methacrylate polymer (U.S. Pat. No. 1,039,498). The former polymer remains slightly crystalline in the fiber and thus causes light scattering at the interface between the core and the cladding, thereby leading to reduction of transmitting properties, whereas the latter polymer has the disadvantages of being insufficient in adhesion to the core and flexibility, and widely variable in properties with temperatures. Boronor fluorine-containing quartz glass is unsatisfactory in flexibility and considerably expensive, thus economically undesirable.
It is an object of the present invention to provide a novel curable composition significantly suitable as a cladding material for optical fibers.
It is another object of the invention to provide a novel curable composition which can overcome the foregoing drawbacks of conventional cladding materials and which is fully satisfactory in all of the properties required of cladding materials.
These and other objects of the present invention will become more apparent from the following description.